Sheet fed thermo form machines have clamp frames which hold a plastic sheet of thermal formable plastic in place during the heating and forming cycles of the machines. The machines can be provided with different sets of clamp frames to facilitate the use of different sized plastic sheets. The clamp frames must be changed when different sized plastic sheets are to be formed. This can take from thirty minutes on a small single station machine and two to three hours on a large rotary thermo forming machine. The changing of the clamp frames is normally done immediately after a job is completed so that the clamp frames are hot which requires considerable undesirable work and time to set up for a new job. Each time that the plastic sheet size is changed for dimensions greater than one half inch, the clamp frames must be changed. This requires the disconnecting of air lines, removing transverse clamp frames, and loosening the two longitudinal clamp frames. The longitudinal clamp frames are moved in or out to new settings to match the size of the new plastic sheet. Different transverse clamp frames are required and mounted on the machine frames. All of the bolts are then tightened and the air lines are reconnected. This process is counterproductive to modern manufacturing techniques that require quick tool changes to reduce the non-productive time of the machine. The tedious adjustment of the clamp frames prohibits profitable short production runs which minimize material and finished product inventories. The adjustable clamp frames of the invention have overcome the disadvantages of the prior clamp frames for thermo form machines by providing a clamp frames that are quickly adjustable to the required dimensions without removing parts, disconnecting air lines, and loosening and tightening nuts and bolts.